1. Field of the Invention
The present invention relates to a micropower instrument of high accuracy to perform ratiometric measurement of milli-ohm level resistances using milliamp-level current. The invention can be used for general environmental monitoring including, but not limited to, corrosion measurement, strain measurement and other monitoring uses that rely on ratiometric comparisons of a milli-ohm sense resistance to a milli-ohm reference resistance.
2. Background of the Invention
Corrosion can lead to failures in infrastructure, machines, and mission critical systems. Such failures are expensive to repair, can lead to lost or contaminated products, can cause environmental damage, and ultimately, can even cause unsafe environments or situations for humans. Decisions regarding the future integrity of a structure or its components depend substantially upon an accurate assessment of the conditions affecting its corrosion and rate of deterioration. Only with accurate information in hand, can an owner or operator make an informed decision as to the type, cost, and urgency of repair or replacement.
Corrosion monitoring is particularly important in areas that cannot be readily inspected visually or are difficult to inspect due to the inherent structural arrangement of a particular device, machine or structure. For example, there may be cavities within vehicles that are generally not accessible because of equipment or other structures that block an opening to the cavity. Nevertheless, corrosion monitoring of such spaces is desirable, and perhaps critical.
One well-known method of monitoring corrosion is the electrical resistance technique. This technique effectively measures material loss, i.e., corrosion, by measuring a change in electrical resistance of a metallic element, which is exposed to a selected environment, with respect to a reference element that is arranged to be immune from that environment""s corrosive effects. While this technique is very popular and has found wide acceptance, the technique requires the availability of electric power. In some cases, power for the electric resistance technique is obtained from a battery. However, since it is often desirable to monitor environments for relatively long periods of time, battery life becomes an issue for these instruments. Accordingly, there is need for a micropower instrument with greatly-improved battery life and accuracy.
The present invention provides a micropower instrument of high accuracy to perform ratiometric measurement of milli-ohm level resistances using milliamp-level current. The invention can be used for general environmental monitoring including, but not limited to, corrosion measurement, strain measurement and other monitoring uses that rely on ratiometric comparisons of a milli-ohm sense resistance to a milli-ohm reference resistance. The apparatus in accordance with the present invention is an analog measurement instrument that consumes an extremely low amount of power (thereby providing longer battery life), has high precision and accuracy, and, in a preferred embodiment, operates on and measures the resistance of a sacrificial electrical resistance coupon. The instrument is able to detect and monitor resistance in the mili ohm range.
In accordance with a corrosion detection implementation of the present invention, corrosion is measured by comparing the resistance of a corroding xe2x80x9ctest couponxe2x80x9d to a protected xe2x80x9creferencexe2x80x9d coupon that was identical or nearly so at the time of manufacture. The resistances of these coupons are very low, typically on the order of a few milliohms. Because low average power consumption is a desirable feature, high-current excitation of the coupons is not an option. On the other hand, low-current excitation results in signals of microvolt magnitude. The present invention was developed in view of the fact that available commercial instruments fail, by a wide margin, to meet low-power requirements for an environmental analysis detection system that is intended to be located or positioned in places that may generally be inaccessible, or that need to be xe2x80x9con-stationxe2x80x9d for long periods of time.
Features of the present invention include, but are not limited to:
the use of AC current to excite the coupons to avoid errors due to DC offset in amplifiers and thermoelectric potentials at various connection points;
the use of a 10:1 current step-up transformer in a drive circuit to gain a tenfold increase in power efficiency in driving the very low-impedance load (the coupons);
the use of very low-noise, low-offset, high-gain instrumentation operational amplifiers in a first signal-processing stage; and
the use of ratiometric measurement by current driving the reference and sensor coupons in series, sensing and signal-processing their responsive voltages, and taking the ratio of these voltages in subsequent digital signal processing.
The present invention can also be employed as a resistive straingage. More specifically, the present invention provides an instrument that can be used to monitor straingages or other resistances that are proportional to strain. The term xe2x80x9cstraingagexe2x80x9d commonly refers to a resistive element that changes resistance with strain. Resistive straingages are routinely used for measurements of strain in structural elements or members. Such straingages have relatively high resistance to aid in ease of instrumentation, but they are relatively delicate, fragile, and require considerable care and skill to affix. They are often configured or arranged as bridges to facilitate ratiometric measurement. The ability to measure very low resistances with a micropower instrument in accordance with the present invention makes it possible to observe strain in a metal structural member (as in a sheetmetal skin) by directly measuring changes in resistance between various points in the strained member itself, obviating the need for straingages.
The features and attendant advantages of the present invention will be more fully appreciated upon a reading of the following detailed description in conjunction with the accompanying drawings.